Impulsivity is a prominent characteristic of both bipolar disorder (BD) (Swann et al., 2003, 2001) and substance abuse disorder (SA) (Moeller and Dougherty, 2002). Furthermore, the comorbidity of BD with SA is a well-documented pattern of psychiatric presentation (e.g., Krishnan, 2005). The strength of this association has been attributed to common underlying biobehavioral processes of impulsivity in BD and SA (Dougherty et al., 2004; Swann et al., 2004). However, each of these psychiatric conditions may also involve separate aspects of impulsivity (Moeller et al., 2001), which underscores the necessity for assessment of multiple characteristics of both trait-dependent and state-dependent impulsive behavior to truly appreciate the contribution of impulsivity to BD and SA (Dougherty et al., 2003b).

Impulsivity Across Bipolar Phase and Substance Abuse

An excellent example of the importance of considering the type of impulsivity assessment within psychiatric disorders is Swann and colleagues' (2004) report on the use of both self-report and laboratory behavioral measures of impulsivity in individuals with comorbid and independent diagnoses of BD and SA compared to controls. The Barratt Impulsiveness Scale (BIS) (Patton et al., 1995) was the self-report measure of trait-dependent aspects of impulsivity, while the Immediate and Delayed Memory Task (IMT/DMT) (Dougherty et al., 2002) was the behavioral measures of state-dependent aspects of impulsivity. The patients with BD were classified into one of four groups based on phase of their BD (manic or euthymic) and presence or absence of SA history (SA+ or SA-) (Swann et al., 2004). These patients were compared to two additional groups recruited from the community: adults with and without a history of SA (SA-only and controls). The result was a comparison of trait-dependent and state-dependent impulsivity between six groups: manic/SA+, manic/SA-, euthymic/SA+, euthymic/SA-, SA-only and controls.

Trait-dependent impulsivity (BIS total scores). While global impulsive behavior of people with euthymic and manic BD has shown essentially identical elevations of trait-dependent impulsivity compared to controls (Swann et al., 2003), the study being examined here compares trait-dependent impulsivity in euthymic patients with and without SA to impulsivity in SA-only individuals and controls (Swann et al., 2004) (Figure 1, left). Levels of trait-dependent impulsive behaviors reported by individuals in the euthymic/SA- and SA-only groups were essentially identical; however, comorbidity of these two disorders (euthymic/SA+) appeared to have an additive effect on trait-dependent impulsive behavior. Collectively, this indicates that elevated trait-dependent impulsivity is dependent on both SA and BD regardless of the phase of illness.

State-dependent impulsivity (IMT). Alternatively, the profile for state-dependent impulsivity provides a different assessment of these groups (Swann et al., 2004) (Figure 1, right). Comparing people with euthymic BD with and without SA to SA-only and controls showed that state-dependent impulsive performance of the euthymic-only patients (euthymic/SA-) presented a profile similar to the controls, while individuals with comorbid euthymic phase BD and SA (euthymic/SA+) presented a profile of performance similar to SA-only. This indicates that elevated state-dependent impulsive behavior is dependent on SA during the euthymic phase of BD. A more detailed examination illustrates that while differentiation of state-dependent impulsivity in the euthymic phase is dependent on comorbidity with SA, the manic phase of BD produces elevated state-dependent impulsive behavior regardless of SA comorbidity (Figure 2). Therefore, elevated state-dependent impulsive behavior is also dependent on the phase of the BD.

These examples demonstrate how measurements using different modes of impulsivity assessment lead to different profiles of impulsive behavior (Swann et al., 2004). The trait-dependent measure showed that euthymic and manic BD, as well as SA impulsivity profiles, are different from controls but not from each other. This trait-dependent assessment also showed that the impulsivity profile is dependent on the comorbidity of these disorders in the euthymic phase. Conversely, the state-dependent measure clearly showed that current impulsive tendencies were dependent both on SA history and BD phase of illness.

These comparisons demonstrate that trait-dependent measures provide a different profile of impulsivity than state-dependent measures in BD and SA and research employing concurrent use of more than one type of impulsivity measure can lead to a more thorough characterization of underlying behavioral mechanisms. Continued and more detailed efforts are needed to provide a more comprehensive view of how various aspects of impulsivity interact within BD and SA and in impulse control disorders in general.